Various types of auditory devices provide persons with different types of hearing loss with the ability to perceive sound or perceive improved sound, or persons having normal hearing to experience different sound perception (e.g. with less noise). Hearing loss may be conductive, sensorineural, or some combination of both conductive and sensorineural. Conductive hearing loss typically results from a dysfunction in any of the mechanisms that ordinarily conduct sound waves through the outer ear, the eardrum, or the bones of the middle ear. Sensorineural hearing loss typically results from a dysfunction in the inner ear, including the cochlea, where sound vibrations are converted into neural signals, or any other part of the ear, auditory nerve, or brain that may process or convey the neural signals.
Persons with some forms of conductive hearing loss may benefit from auditory devices such as acoustic hearing aids or vibration-based auditory devices. An acoustic hearing aid typically includes a small microphone to detect sound, an amplifier to amplify certain portions of the detected sound, and a small speaker to transmit the amplified sounds into the person's ear. Vibration-based auditory devices typically include a small microphone to detect sound and a vibration mechanism to apply vibrations corresponding to the detected sound directly or indirectly to a person's bone or teeth, for example, thereby causing vibrations in the person's inner ear and bypassing the person's auditory canal and middle ear. Vibration-based auditory devices include, for example, bone-anchored devices, direct acoustic cochlear stimulation devices, or other vibration-based devices. A bone-anchored device typically utilizes a surgically implanted mechanism or a passive connection through the skin or teeth to transmit vibrations corresponding to sound via the skull. A direct acoustic cochlear stimulation device also typically utilizes a surgically implanted mechanism to transmit vibrations corresponding to sound, but bypasses the skull and more directly stimulates the inner ear. Other non-surgical vibration-based auditory devices may use similar vibration mechanisms to transmit sound via direct or indirect vibration of teeth or other cranial or facial bones or structures. Persons with severe to profound sensorineural hearing loss may benefit from surgically implanted auditory devices (prostheses), such as cochlear implants, auditory brainstem implants, or auditory midbrain implants. For example, a cochlear implant can provide a person having sensorineural hearing loss with the ability to perceive sound by stimulating the person's auditory nerve via an array of electrodes implanted within the cochlea. A component of the cochlear implant detects sound waves, which are converted into a series of electrical stimulation signals that are delivered to the person's cochlea via the array of electrodes. Auditory brainstem implants can use technology similar to cochlear implants, but instead of applying electrical stimulation to a person's cochlea, auditory brainstem implants apply electrical stimulation directly to a person's brainstem, bypassing the cochlea altogether. Electrically stimulating auditory nerves in a cochlea with a cochlear implant or electrically stimulating a brainstem may enable persons with sensorineural hearing loss to perceive sound. Further, some persons may benefit from auditory devices that combine one or more characteristics of acoustic hearing aids, vibration-based auditory devices, cochlear implants, and auditory brainstem implants to enable the person to perceive sound. Such auditory devices can be referred to generally as hybrid auditory devices.
Common among some or all the above-described auditory devices is the need to determine the stimulus to provide to the auditory devices' stimulation mechanism/device (electrode, vibrator, speaker, etc.), so that the user of the auditory device is able to hear important sounds (information) at a loudness that is perceptible, yet comfortable for the user. This requires, first, that the auditory device be properly fit to the user, so that one or more stimulation channels, for example, provide appropriate maximum and minimum levels of stimulation to the user. For example, an acoustic hearing aid should be fit so that the hearing aid's speaker preferably does not cause discomfort to the user in the presence of loud ambient sounds, but still allows the user to hear quiet ambient sounds. For electrical, rather than acoustic stimulation, fitting typically refers to choosing an acceptable range of current levels to be provided to one or more stimulation electrodes, or a stimulus signal to be provided to a vibration mechanism or other source of stimulation.
In addition to fitting, how the auditory device initially handles the incoming ambient sounds may also be important. Noise reduction techniques are commonly employed in auditory devices to attenuate parts of the signal that are determined to be noise, while retaining the target information content of the signal. Compression and expansion techniques are also commonly employed in auditory devices to amplify or attenuate signals that are too soft or too loud to improve listening comfort and speech understanding.
While noise reduction can decrease the loudness of masking noise, it can also, to a lesser extent, decrease the loudness of the target talker. It also has no inherent limits, so in very noisy environments typical noise reduction schemes remove much of the noise signal, which reduces listening quality. While compression and expansion systems can improve listening comfort in loud or soft environments, they can also have negative effects on listening quality. For example, in the case of compressors that attenuate short, loud sounds, they may also attenuate the background during and after the short loud sound creating the perception of a transient pumping noise, which reduces listening quality.
Thus, even if an auditory device is properly fit to its user and includes some form of noise reduction and compression and expansion algorithms, listening in noisy or loud or soft environments can result in relatively poorer speech quality and intelligibility when listening in real-world environments.